Green’s Dictionary of Slang

secko n.

also secco, sekkie
[SE sex + -o sfx (3)]

(Aus., usu. prison) a sexual pervert.

[Aus]R. Park Poor Man’s Orange 38: ‘Just look at that dirty ole secko, will you?’ he said disgustedly, and scooping up a stone he ran after it, yelling, ‘Merv, Merv, the rotten old perv.’.
[Aus] ‘Whisper All Aussie Dict.’ Kings Cross Whisper (Sydney) xxxix 4/5: sexos or seckos: Prisoners sentenced for sex crimes against young persons. The most despised of all prisoners.
[SA]C. Hope Ducktails in Gray Theatre Two (1981) 44: bo-bo Hell, I fancy that dark-haired goosey. [...] D’you check the way she sort of looked at me? I think I’m in there. howellsie Kap it uit, ou sekkie!
[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. [Internet] Secco. A sexual offender. Term generalises to someone who is obsessed with sex.
[Aus]G. Seal Lingo 77: Some less than complimentary examples include secko for a sex maniac or pervert, also sicko for someone whose interests are off-centre.